Soft start circuit of LLC constant current LED power supply

Abstract

The invention discloses a soft start circuit of an LLC constant-current LED power supply, which solves the problems of output current overshoot and magnetic core saturation caused by resonance control adopted by an LED driving power supply in the prior art, and comprises a boost unit connected with a power supply input end, the boost unit is connected with an LLC resonance unit, the LLC resonance unit is connected with a driving control unit, and the driving control unit is connected with the LED driving power supply. The LLC resonance unit is connected with a sampling resistor and a soft start control unit, the sampling resistor performs constant current detection on the LLC resonance unit, the soft start control unit is connected with the sampling resistor and performs voltage division operation on sampling voltage of the sampling resistor, and the LLC resonance unit is connected with the output unit. The sampling resistor can be subjected to voltage division operation, the peak current of the resonance element during starting is remarkably reduced, magnetic core saturation caused by overlarge peak current is avoided, and the current stress to other components is also reduced.

Description

A soft-start circuit for an LLC constant current LED power supply Technical Field

[0001] This invention relates to the field of LED driver power supply technology, and in particular to a soft-start circuit for an LLC constant current LED power supply. Background Technology

[0002] With the widespread application of LED driver power supplies in the market and the rise in global energy costs, the operating conditions for LED power supplies are constantly improving, and the requirements for power efficiency levels are becoming increasingly stringent. In recent years, in pursuit of high-efficiency power supplies, soft-switching technology has been widely used in the field of LED driver power supplies. The most common type is the LLC topology, which has a gain of 1 and the highest efficiency at the resonant frequency. The input voltage is effectively applied to the output load, and the voltage across the resonant cavity, such as other inductors and capacitors, is equivalent to zero, thus avoiding significant losses in these resonant components.

[0003] For example, the existing patent CN109661072A describes an LLC resonant converter, an LED driving circuit, and its control method. The LLC resonant converter includes: a first transformer; a first bipolar transistor and a second bipolar transistor connected in series and having an intermediate node, with the primary winding connected to the intermediate node; a control circuit connected to the base of at least one of the first and second bipolar transistors; and a resonant element connected to the primary winding to form a resonant circuit. The first and second bipolar transistors use the induced current of the resonant current flowing through the resonant element as the driving current. The control circuit periodically short-circuits the driving current according to a clock signal, causing the switching cycle of the first and second bipolar transistors to follow the clock signal. This resonant converter uses a control circuit to control the resonant frequency, which simplifies the control circuit and reduces circuit cost. However, the LED driver power supply resonant control method suffers from abnormal harmonic control, leading to output current overshoot, poor voltage stability control, and high latency, failing to meet user needs. Summary of the Invention

[0004] The purpose of this invention is to solve the problem that the existing LED driver power supply uses resonant control, which leads to output current overshoot and core saturation. This invention provides a soft-start circuit for an LLC constant current LED power supply, which can reduce the voltage on the sampling resistor in stages, significantly reducing the peak current of the resonant element when the LED power supply starts. The circuit design is flexible, and the number of components is small, resulting in low cost.

[0005] To achieve the above objectives, the present invention adopts the following technical solution: a soft-start circuit for an LLC constant current LED power supply, comprising: a boost unit connected to the power input terminal, wherein the boost unit rectifies the input voltage, performs PFC calibration, and then boosts the voltage, and connects the output voltage to an LLC resonant unit; the LLC resonant unit is connected to a drive control unit, which performs frequency control on the LLC resonant unit; the LLC resonant unit is connected to a sampling resistor and a soft-start control unit, the sampling resistor performs constant current detection on the LLC resonant unit; the soft-start control unit is connected to the sampling resistor and performs voltage division on the sampling voltage of the sampling resistor; and the LLC resonant unit is connected to an output unit.

[0006] The soft-start circuit of the LLC constant current LED power supply provided by this invention can perform voltage division operation on the sampling resistor, significantly reducing the peak current of the resonant element during startup, avoiding core saturation due to excessive peak current, and also reducing current stress on other components.

[0007] Preferably, the soft-start control unit includes a MOSFET, the drain of which is connected to a first voltage divider resistor, which is connected to a sampling resistor. The source of the MOSFET is grounded through a second voltage divider resistor. The gate of the MOSFET is connected to the auxiliary power supply terminal of the LLC resonant unit through a soft-start resistor and a unidirectional discharge diode. The gate of the MOSFET is connected to a soft-start capacitor and a Zener diode.

[0008] Preferably, the voltage after voltage division is the ratio of the voltage dividing current to the total voltage dividing resistance, the voltage dividing current is the product of the original voltage and the resistance of the first voltage dividing resistor, and the total voltage dividing resistance is the sum of the resistance of the first voltage dividing resistor and the resistance of the second voltage dividing resistor.

[0009] Preferably, the LLC resonant unit includes a resonant capacitor, a resonant inductor, and a magnetizing inductor. One end of the resonant capacitor is connected to the drive control unit, the other end of the resonant capacitor is connected to one end of the resonant inductor, the other end of the resonant inductor is connected to the sampling resistor, and the magnetizing inductor is connected to the soft-start control unit. The LLC resonant unit reduces the boosted DC voltage to a voltage suitable for the LED load to operate.

[0010] Preferably, the boost unit includes a rectifier bridge, the AC input terminal of which is connected to the power input terminal, the DC output terminal of which is connected to one end of an inductor L1, the other end of which is connected to a chip U1 and a boost diode, the chip U1 is connected to a filter capacitor, and the filter capacitor is connected to the boost diode and outputs a DC voltage to the LLC resonant unit.

[0011] Preferably, the drive control unit includes a drive transformer T1 and a switching transistor Q1. The switching transistor is grounded through a lower freewheeling diode and connected to the output unit through an upper freewheeling diode. The drive control unit drives the LLC resonant unit to operate and adjusts the output load by switching the transistor at high frequency.

[0012] Preferably, the device also includes a chip U2, the CS terminal of which is connected to the drain of the MOS transistor, the CS terminal of which is connected to the sampling resistor through a second voltage divider resistor, and the VDD terminal of which is connected to the VFB terminal of which is connected through a resistor.

[0013] Preferably, the output unit includes a transformer secondary winding, which is connected to the positive terminals of rectifier diodes D3 and D5 respectively. The negative terminals of rectifier diodes D3 and D4 are connected and then connected to one end of capacitor C4. The other end of capacitor C4 is connected to the transformer secondary winding. The output unit uses full-wave rectification output.

[0014] Preferably, the resonant inductor is the leakage inductance of the transformer, and the magnetizing inductance is the magnetizing inductance of the primary winding of the transformer.

[0015] Preferably, the sampling resistor includes a first sampling resistor and a second sampling resistor, which are connected in parallel.

[0016] Therefore, this invention has the following beneficial effects: by dividing the original voltage of the sampling resistor, the current value previously set by the user is achieved, thereby avoiding excessive resonant inductor current during power-on, thus realizing a soft start and enabling rapid discharge during power-off. It also achieves autonomous control of the resonant frequency of the high-power LED driver, improving the working performance of the LED driver, and eliminating current overshoot during power-on and power-off under full load. Attached Figure Description

[0017] Figure 1 is a structural block diagram of the soft start circuit in this invention.

[0018] Figure 2 is a schematic diagram of the circuit structure of the boost unit in this invention.

[0019] Figure 3 is a schematic diagram of the circuit structure of the remaining circuit units in this invention.

[0020] In the diagram: 1. Boost unit; 2. Drive control unit; 3. LLC resonant unit; 4. Output unit; 5. Sampling resistor; 6. Soft start control unit. Detailed Implementation

[0021] The technical solutions of the present invention and how they solve the above-mentioned technical problems will be described in detail below with reference to the accompanying drawings and specific embodiments. The following specific embodiments can be combined with each other, and the same or similar concepts or processes may not be described again in some embodiments.

[0022] Example 1: This example provides a soft-start circuit for an LLC constant current LED power supply, as shown in Figure 1. It includes: a boost unit 1, a drive control unit 2, an LLC resonant unit 3, a sampling resistor 5, a soft-start control unit 6, and an output unit 4. The input terminal of the boost unit is connected to the power input terminal, the output terminal of the boost unit is connected to the input terminal of the LLC resonant unit, the drive control unit is connected to the input terminal of the LLC resonant control unit, the LLC resonant unit is connected to the sampling resistor, the soft-start control unit, and the output unit, and the soft-start control unit is connected to the LLC resonant unit and the sampling resistor.

[0023] The boost circuit rectifies the input voltage, performs PFC calibration, and then boosts the voltage to input the output voltage to the LLC resonant unit. The drive control unit is connected to the LLC resonant unit and performs frequency control on the LLC resonant unit. The LLC resonant unit is responsible for energy conversion and transfer. The sampling resistor is used to sample the current of the LLC resonant unit for constant current detection.

[0024] During operation: During the power-on startup phase, the LLC chip in the drive control circuit initially operates at minimum current. Then, it uses the supply voltage of the auxiliary winding to charge the soft-start capacitor through the soft-start resistor. This, in turn, drives the MOSFET to conduct. Once the MOSFET is turned on, it divides the sampling voltage across the sampling resistor to achieve the user-defined output voltage. Compared to existing technologies, this circuit design is flexible, requires fewer components, is inexpensive, and offers stable functionality.

[0025] This embodiment provides a soft-start circuit for an LLC constant current LED power supply, which can perform voltage division operation on the sampling resistor, significantly reducing the peak current of the resonant element during startup, avoiding core saturation due to excessive peak current, and also reducing current stress on other components.

[0026] The following examples and specific application scenarios further illustrate the technical solution and effects of the present invention. The following examples are explanations of the present invention, but the present invention is not limited to the following examples.

[0027] When applying an LLC topology to an LED driver power supply, during the LLC startup process, the primary voltage is clamped to 0 by the secondary winding voltage. At this startup, all voltage is applied to the resonant inductor. To reduce the inductor's peak current, the chip can only increase the switching frequency. However, even at the highest startup frequency, if the input voltage is high enough, the inductor's peak current is still very large, leading to output current overshoot and poor voltage stability control. If the voltage across the sampling resistor can be reduced in stages, the peak inductor current during startup can be significantly reduced, preventing core saturation due to excessive peak current and reducing current stress on other components. Therefore, this embodiment provides a soft-start circuit for an LLC constant current LED power supply that achieves the aforementioned technical effects.

[0028] Specifically, this embodiment provides a soft-start control unit, which includes a diode D20 connected to the output terminal of the auxiliary winding of transformer T2, a soft-start resistor R34, a soft-start capacitor C24, a sampling voltage divider resistor R30, a second voltage divider resistor R15, and a MOSFET Q7 connected to the control sampling resistor. The drain of MOSFET Q7 is connected to the first voltage divider resistor, which is connected to the sampling resistor. The source of MOSFET Q7 is grounded through the sampling voltage divider resistor R30. One path of the gate of MOSFET Q7 is connected to the auxiliary power supply terminal of the LLC resonant unit through the soft-start resistor R34 and the unidirectional discharge diode D21. The other path of the gate of MOSFET Q7 is connected to ground through the parallel Zener diode ZD2 and the soft-start capacitor C24.

[0029] In this embodiment, the soft-start control unit also includes chip U2. The CS terminal of chip U2 is connected to the drain of the MOSFET. The CS terminal of chip U2 is also connected to the sampling resistor through a second voltage divider resistor. The VDD terminal of chip U2 is connected to the VFB terminal of chip U2 through a resistor. Specifically, pin 1 of chip U2 is connected to the drain of MOSFET Q7, and pin 1 is connected to the first sampling resistor and the second sampling resistor to ground through a voltage divider resistor R15. The auxiliary winding of transformer T2 is connected to the anode of diode D20, and the cathode of diode D20 is connected to resistor C23 and resistor R33 and grounded.

[0030] During operation, in the power-on phase: the auxiliary winding establishes a stable voltage NUo through diode D20 and capacitor C23, which is related to the output voltage Uo by a certain turns ratio N. This voltage drives the MOSFET Q7 to switch via the soft-start resistor R34. At startup, this voltage hasn't reached the gate threshold voltage of MOSFET Q7 and it hasn't turned on. At this time, a very small current charges the soft-start capacitor C8 through the soft-start resistor R34. The LC resonant loop then starts up using the current across the fixed sampling resistor. As the gate voltage of MOSFET Q7 exceeds the Vth threshold voltage, MOSFET Q7 turns on. The sampling voltage at the CS terminal of chip U2 is then divided by the first voltage divider resistor R15 and the second voltage divider resistor R30. The divided voltage is the product of the original CS terminal voltage Ucs and the resistance of the second voltage divider resistor R30, and then the ratio of this product to the sum of the resistances of the first and second voltage dividers (R15 + R30). By dividing the original voltage across the sampling resistor, the user-set current value is achieved, thus preventing excessive resonant inductor current during startup and achieving a soft start. Meanwhile, during the power-off phase, the soft-start capacitor C8 discharges rapidly through the unidirectional discharge diode D21 and resistor R33.

[0031] The boost unit is the pre-amplifier of the LLC resonant unit, ensuring that the resonant unit can support a wide range of input voltages. The boost unit primarily boosts the input voltage and performs PFC correction to meet the gain requirements of the subsequent LLC circuit under different loads. The boost unit includes a rectifier bridge BD1, inductor L1, chip U1, filter capacitor C9, and boost diode D1. The AC input terminal of rectifier bridge BD1 is connected to the AC voltage output terminal, and the DC output terminal of rectifier bridge BD1 is connected to inductor L1. Inductor L1 is connected to both chip U1 and boost diode D1. Both chip U1 and boost diode D1 are connected to filter capacitor C9, which is connected to the LLC resonant control unit.

[0032] The drive control unit is the unit in chip U1 used to drive the LLC resonant unit. The drive control unit consists of a drive transformer T1, a switching transistor Q1, an upper freewheeling diode D2, and a lower freewheeling diode D4. The drive transformer T1 is connected to the switching transistor Q1, which is grounded through the lower freewheeling diode D4. The switching transistor Q1 is also connected to the output unit through the upper freewheeling diode D2. By switching the transistor Q1 at high frequency, the output load is adjusted to operate at a suitable gain point.

[0033] The LLC resonant unit includes a resonant capacitor C8, a resonant inductor composed of the leakage inductance of transformer T2, and a magnetizing inductor of the primary winding of transformer T2. One end of the resonant capacitor is connected to the drive control unit, and the other end of the resonant unit is connected to one end of the resonant inductor. The other end of the resonant inductor is connected to the sampling resistor, and the magnetizing inductor is connected to the soft-start control unit. The specific parameters and component selection of the LLC resonant circuit need to be selected according to the actual situation. The LLC resonant unit is directly connected to the output of the boost unit, which can reduce the boosted DC voltage to a voltage suitable for LED load operation.

[0034] The output unit includes the secondary winding of transformer T2, rectifier diodes D3 and D5, and output electrolytic capacitor C4. The secondary winding of transformer T2 is connected to the positive terminals of rectifier diodes D3 and D5 respectively. The negative terminals of rectifier diodes D3 and D4 are connected to one end of capacitor C4, and the other end of capacitor C4 is connected to the secondary winding of the transformer. The output unit circuit uses a full-wave rectification output method.

[0035] In this embodiment, the sampling resistor includes a first sampling resistor and a second sampling resistor, which are connected in parallel. One end of the sampling resistor is grounded, and the other end is connected to the soft start control unit and the LLC resonant unit, respectively.

[0036] Therefore, the soft-start circuit of the LLC constant current LED power supply provided in this embodiment has the following beneficial effects: by dividing the original voltage of the sampling resistor, the current value previously set by the user is achieved, thereby avoiding excessive resonant inductor current during power-on, thus realizing a soft start and enabling rapid discharge during power-off. It achieves autonomous control of the resonant frequency of the high-power LED driver power supply, improving the working performance of the LED driver power supply, and eliminating current overshoot during power-on and power-off under full load conditions.

[0037] Example 2: Specifically, based on Example 1, this example adds a specific circuit structure for the soft-start circuit and autonomous regulation of the resonant current of the high-power LED driver power supply, so that there is no current overshoot when starting or stopping the power supply under full load.

[0038] This embodiment provides a soft-start circuit for an LLC constant current LED power supply, including: a boost unit, a drive control unit, an LLC resonant unit, a sampling resistor, a soft-start control unit, and an output unit. The boost unit is connected to the input terminal, rectifies the input voltage, performs PFC calibration, and then boosts the voltage, connecting the output voltage to the LLC resonant unit. The drive control unit is connected to the LLC resonant unit and performs frequency control on the LLC resonant unit. The LLC resonant unit is connected to the sampling resistor, the soft-start control unit, and the output unit; it is responsible for energy conversion and transfer. The sampling resistor is used to sample the current of the LLC resonant unit for constant current detection. The soft-start control unit is connected to the LLC resonant unit and the sampling resistor.

[0039] Specifically, the circuit structure of the soft starter is shown in Figures 2 and 3.

[0040] As shown in Figure 2, the boost unit includes a fuse F1, a capacitor CX1, a filter LF1, a rectifier bridge BD1, an inductor L2, a resistor R4, capacitors C1, CC1, C7, C9, C13, and C14, an inductor L1, diodes D1, D7, D8, and DS1, resistors R12, R18, and R19, and resistors R9, R7, and R20. Capacitor CX1 is connected in parallel with the input terminal of filter LF1. The fuse is connected to capacitor CX1. The output terminal of filter LF1 is connected to the AC input terminal of rectifier bridge BD1. The DC output terminal of rectifier bridge BD1 is connected to inductor L2. The first terminal is connected to the first terminal of resistor R4. The second terminal of resistor R4 is connected to one terminal of capacitor CC1, the second terminal of inductor L2, the first terminal of capacitor C1, the positive terminal of diode DS1, the fourth terminal of inductor L1, and the first terminal of resistor R9. The second terminal of capacitor C1 and the negative terminal of diode DS1 are both connected to the negative terminal of diode D1. The second terminal of resistor R19 is connected to the first terminal of resistor R17. The second terminal of resistor R17 is connected to the power supply VCC terminal. The negative terminal of diode D8 is connected to the power supply VCC terminal. The positive terminal of diode D8 is connected to the negative terminal of diode D7 and the first terminal of capacitor C7. The positive terminal of diode D7 is grounded. The second terminal of capacitor C7 is connected to the seventh terminal of inductor L1. The tenth terminal of inductor L1 is grounded. The second terminal of inductor L1 is connected to the positive terminal of diode D1 and the four DRAIN terminals (pins 5 to 8) of chip U1. The CS terminal (pin 4) of chip U1 is connected to the 9th terminal of resistor R1 and the first terminal of resistor R20. The VDD terminal (pin 3) of chip U1 is connected to the power supply VCC terminal and the first terminal of capacitor C14. The FB terminal (pin 1) of chip U1 is connected to the first terminal of capacitor C13, the first terminal of resistor R23, and the first terminal of resistor R18. The second terminals of resistor R19, resistor R20, capacitor C14, capacitor C13, resistor R23, and the GND terminal (pin 2) of chip U1 are all grounded. The other terminal of resistor R18 is connected to the first terminal of resistor R12. The other terminal of resistor R12 is connected to the negative terminal of diode D1 and the first terminal of capacitor C9. The second terminal of capacitor C9 is grounded. The first terminal of capacitor C9 is connected to the LC resonant unit.

[0041] During operation, the boost unit rectifies the input AC voltage through the rectifier bridge BD1, then converts it into a higher DC voltage through chip U1, inductor L1, and boost diode D1. After filtering and stabilization by filter capacitor C9, the DC voltage is supplied to the subsequent LLC resonant unit. The specific parameters of the boost circuit can be designed according to actual requirements.

[0042] As shown in Figure 3, the soft-start control unit includes a MOSFET Q7, resistors R15, R30, R33, and R34, capacitors C8 and C23, diode D21, and diode ZD2. The source of MOSFET Q7 is connected to ground via the voltage divider resistor R30. The gate of MOSFET Q7 is connected to the auxiliary power supply terminal via the soft-start resistor R34 and the positive terminal of the unidirectional discharge diode D21. At the same time, it is connected to ground via the parallel Zener diode ZD2 and the soft-start capacitor C34.

[0043] Specifically: The drain of MOSFET Q7 is connected to the first terminal of resistor R15, the second terminal of resistor R15 is connected to the sampling resistor, the source of MOSFET Q7 is connected to the first terminal of resistor R30, the second terminal of resistor R30 is grounded, the gate of MOSFET Q7 is connected to the anode of diode D21, the first terminal of capacitor C8, the cathode of diode ZD2, and the first terminal of resistor R34, the other terminal of capacitor C8 is grounded, the anode of diode ZD2 is grounded, the other terminal of diode D21 and resistor R34 are connected to the cathode of diode D20, the first terminal of resistor R33, and the first terminal of capacitor C23, the second terminals of resistor R33 and capacitor C23 are grounded, the anode of diode D20 is connected to terminal 1 of transformer T2, and terminal 2 of transformer T2 is grounded.

[0044] In this embodiment, the soft start control unit also includes chip U2, resistors R14, R13, R22, R29, R16, R21, capacitors C10, C11, and C12, and diodes D19, D6, and ZD1. Pin 6 of chip U2 is connected to the VFB terminal, pin 3 of chip U2 is connected to the TX1 terminal, pin 4 of chip U2 is connected to the TX2 terminal, pin 2 of chip U2 is connected to the GND terminal, and pin 5 of chip U2 is connected to the VDD terminal, which is connected to VFB via resistor R29.

[0045] Specifically, the CS terminal (pin 1) of chip U2 is connected to the first terminal of resistor R15. The VDD terminal (pin 5) of chip U2 is connected to the first terminals of resistors R22, R14, and R29 respectively. The second terminal of resistor R29 is connected to the VFB terminal (pin 6) of chip U2. The second terminal of resistor R14 is connected to the anode of diode ZD1, and the cathode of diode ZD1 is connected to the power supply VCC terminal. The second terminal of resistor R22 is grounded, and the second terminal of capacitor C10 is grounded. The VFB terminal of chip U2 is connected to the first terminals of capacitor C12, R21, and R16 respectively. The second terminals of resistor R12 and capacitor C12 are both grounded. The second terminal of resistor R16 is connected to the cathode of diode D6 and the anode of diode D19 respectively. The cathode of diode D19 is connected to the other end of resistor R13. The anode of diode D6 is connected to the anode of diode D20 and then connected to terminal 1 of transformer T2.

[0046] The sampling resistors include resistors R10 and R11. The first terminals of resistors R10 and R11 are both connected to the second terminal of resistor R15. The second terminals of resistors R10 and R11 are both grounded.

[0047] The drive control unit includes a switching transistor Q1, a transformer T1, resistors R1, R2, R5, R7, and R8, diodes D2 and D4, and a capacitor C6. The C1 terminal (pin 8) of the switching transistor Q1 is connected to the first terminal of capacitor C9, the cathode of diode D2, and the first terminal of resistor R1. The second terminal of resistor R1 is connected to the first terminal of resistor R2. The second terminal of resistor R2 is connected to the first terminal of resistor R7, the first terminal of capacitor C8, and the CT2 terminal of transformer T1B. The CT1 terminal (second terminal) of transformer T1B is connected to the E1 terminal (pin 6) of the switching transistor Q1 and the anode of diode D2. Connect the two E1 terminals (pins 6 and 7) of the switching transistor Q1 to the negative terminal of diode D4 and the first terminal of capacitor C6. Connect the B1 terminal (pin 1) of the switching transistor Q1 to the first terminal of resistor R3. Connect the second terminal of resistor R3 to the first terminal of transformer T1A. Connect the B2 terminal (pin 3) of the switching transistor Q1 to the first terminal of resistor R5. Connect the second terminal of resistor R5 to the 8th terminal of transformer T1A. Connect the 7th terminal of transformer T1A, the E2 terminal (pin 4) of the switching transistor Q1, the positive terminal of diode D4, the second terminal of capacitor C6, and the second terminal of resistor R8 to ground. Connect the first terminal of resistor R8 to the second terminal of resistor R7.

[0048] The LLC resonant unit includes a capacitor C8 and the primary winding of a transformer T2. The second terminal of the capacitor C8 is connected to the fifth terminal of the transformer T2, the fourth terminal of the transformer T2 is connected to the first terminal of the resistor R10, and the second terminal of the transformer T2 is grounded.

[0049] The output unit includes the secondary winding of transformer T2, diodes D3 and D5, resistor R6, and capacitor C4. Terminal 10 of transformer T2 is connected to the anode of diode D3. The cathode of diode D3 is connected to the cathode of diode D5, the first terminal of resistor R6, and the first terminal of capacitor C4. The anode of diode D5 is connected to terminal 6 of transformer T2. Terminals 9 and 7 of transformer T2 are connected. The second terminals of resistor R6 and capacitor C4 are both connected to terminal 9 of transformer T2.

[0050] In this embodiment, chip U1 is a BP2636DG chip, chip U2 is a RED2432B chip, and switch Q1 is a RED3032 switch.

[0051] This embodiment provides a soft-start circuit for an LLC constant current LED power supply. During the power-on startup phase, the LLC chip initially operates at minimum current. Then, it uses the supply voltage of the auxiliary winding to charge the soft-start capacitor through the soft-start resistor, which in turn drives the MOSFET to conduct. After the MOSFET is turned on, it performs a voltage divider operation on the sampling voltage of the sampling resistor, thereby achieving the user-defined output current. Compared with existing technologies, this circuit design is flexible, requires fewer components, is low-cost, and has stable functionality. The advantages of this solution are: it achieves soft-start by changing the voltage divider on the chip's current sampling CS pin; the circuit is simple; it compensates for the lack of built-in soft-start functionality in the chip itself; and it also solves the problem of only being able to achieve soft-start using the dimming pin of a dimming IC.

[0052] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the present invention in any way. Other variations and modifications are possible without departing from the technical solutions described in the claims.

Claims

1. A soft-start circuit for an LLC constant current LED power supply, characterized in that, include: A boost unit is connected to the power input terminal. The boost voltage is connected to an LLC resonant unit. The LLC resonant unit is connected to a drive control unit. The LLC resonant unit is connected to a sampling resistor and a soft-start control unit. The sampling resistor performs constant current detection on the LLC resonant unit. The soft-start control unit is connected to the sampling resistor and performs voltage division operation on the sampling voltage of the sampling resistor. The LLC resonant unit is connected to the output unit.

2. The soft-start circuit for an LLC constant current LED power supply according to claim 1, characterized in that, The soft-start control unit includes a MOSFET, the drain of which is connected to a first voltage divider resistor, which is connected to a sampling resistor. The source of the MOSFET is grounded through a second voltage divider resistor. The gate of the MOSFET is connected to the auxiliary power supply terminal of the LLC resonant unit through a soft-start resistor and a unidirectional discharge diode. The gate of the MOSFET is connected to a soft-start capacitor and a Zener diode.

3. The soft-start circuit for an LLC constant current LED power supply according to claim 2, characterized in that, The voltage after voltage division is the ratio of the voltage dividing current to the total voltage dividing resistance. The voltage dividing current is the product of the original voltage and the resistance of the first voltage dividing resistor. The total voltage dividing resistance is the sum of the resistance of the first voltage dividing resistor and the resistance of the second voltage dividing resistor.

4. The soft-start circuit for an LLC constant current LED power supply according to claim 1, 2, or 3, characterized in that, The LLC resonant unit includes a resonant capacitor, a resonant inductor, and a magnetizing inductor. One end of the resonant capacitor is connected to the drive control unit, the other end of the resonant capacitor is connected to one end of the resonant inductor, the other end of the resonant inductor is connected to the sampling resistor, and the magnetizing inductor is connected to the soft start control unit.

5. A soft-start circuit for an LLC constant current LED power supply according to claim 1, 2, or 3, characterized in that, The boost unit includes a rectifier bridge, the AC input terminal of which is connected to the power input terminal, and the DC output terminal of which is connected to one end of an inductor L1. The other end of the inductor L1 is connected to a chip U1 and a boost diode. The chip U1 is connected to a filter capacitor. The filter capacitor is connected to the boost diode and outputs a DC voltage to the LLC resonant unit.

6. The soft-start circuit for an LLC constant current LED power supply according to claim 1, 2, or 3, characterized in that, The drive control unit includes a drive transformer T1 and a switching transistor Q1. The switching transistor Q1 is grounded through a lower freewheeling diode and connected to the output unit through an upper freewheeling diode. The drive control unit drives the LLC resonant unit to work and adjusts the output load by switching the high-frequency switching transistor Q1.

7. A soft-start circuit for an LLC constant current LED power supply according to claim 2 or 3, characterized in that, It also includes chip U2, whose CS terminal is connected to the drain of the MOS transistor, whose CS terminal is connected to the sampling resistor through a second voltage divider resistor, and whose VDD terminal is connected to the VFB terminal of chip U2 through a resistor.

8. The soft-start circuit for an LLC constant current LED power supply according to claim 1, 2, or 3, characterized in that, The output unit includes a transformer secondary winding, which is connected to the positive terminals of rectifier diodes D3 and D5 respectively. The negative terminals of rectifier diodes D3 and D4 are connected and then connected to one end of capacitor C4. The other end of capacitor C4 is connected to the transformer secondary winding. The output unit uses full-wave rectification output.

9. The soft-start circuit of an LLC constant current LED power supply according to claim 4, characterized in that, The resonant inductance is the leakage inductance of the transformer, and the magnetizing inductance is the magnetizing inductance of the primary winding of the transformer.

10. A soft-start circuit for an LLC constant current LED power supply according to claim 1, 2, or 3, characterized in that, The sampling resistor includes a first sampling resistor and a second sampling resistor, which are connected in parallel.

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